As a readout type optical disc for recording multimedia data, CD-ROM, etc., have been put to practical use. A CD-ROM is a medium having a diameter of 12 cm, a thickness of 1.2 mm, a track pitch of 1.6 .mu.m and a recording capacity of 540 Mbytes on one side. A digital video disc (DVD) which can record video data by utilizing compression technologies, such as MPEG (Moving Picture Experts Group) type compression technology, is under development. However, when video data in the MPEG-2 standard is to be recorded onto a current CD-ROM at a data transfer rate of 4 Mbps, only approximately 20 minutes of video data can be recorded. This means that the current CD-ROM medium is insufficient for recording a movie of about 120 minutes.
To solve this problem, technology for increasing recording density to several times that of current optical discs is under development. In this specification, the recording density equivalent to that of the CD-ROM is referred to as "standard density."
For example, an SD (Super Density) with a diameter of 12 cm (i.e., the same diameter as that of the CD-ROM), which can record approximately 5 Gbytes of data on one side, has been proposed. The SD has a track pitch of approximately 0.73 .mu.m, a shortest pit length of approximately 0.4 .mu.m, and employs an efficient modulation mode. The thickness of the SD is 0.6 mm. When two SDs are adhered together, back to back, the combined piece can record as much as approximately 10 Gbytes of data, which is equivalent to 240 minutes of movie data.
An HDMCD (High Density Multimedia Compact Disc) with a diameter of 12 cm (i.e., the same diameter as that of the CD-ROM), which can record approximately 3.7 Gbytes of data on one side, has been proposed. The HDMCD has a track pitch of approximately 0.84 .mu.m and a shortest pit length of approximately 0.45 .mu.m . The thickness of the HDMCD is 0.6 mm.
Art relating to embodiments of the present invention has been disclosed in Japanese Unexamined Patent Publication No. 7-57271.
In Japanese Unexamined Patent Publication No. 6-215406, an optical pickup is disclosed which can focus the respective beam spots upon the respective information-bearing surfaces of two different types of optical discs having different substrate thicknesses.
In Japanese Unexamined Patent Publication No. 5-303766, an optical pickup is disclosed which corrects the coma effect caused by the difference in substrate thickness of the optical discs without changing the focal length by advancing an aspherical optical element having no refracting power into a collimated light beam or withdrawing the optical element therefrom according to the thickness of the loaded optical disc. In this arrangement, the pickup can focus the respective beam spots upon the respective information-bearing surfaces of the respective optical discs having different substrate thicknesses.
In Japanese Unexamined Patent Publication No. 6-259804, an apparatus is disclosed which is equipped with a laser diode for reproducing the standard density CD with a thick substrate and a laser diode for recording and reproducing the high density optical disc with a thin substrate and can focus the laser beam output from the laser diode selected according to the optical disc upon the information-bearing surface through a common optical system.
In order to read information from the high density optical disc, the spot diameter of the laser beam focused upon the information-bearing surface must be reduced to approximately 0.9 .mu.m . In order to reduce the spot diameter to such extent, the wavelength of the laser beam must be shortened or the numerical aperture NA of the objective lens must be increased. However, if the numerical aperture NA of the objective lens is increased, the coma increases in proportion to the third power of the numerical aperture NA of the objective lens. For this reason, if the laser beam which falls upon the substrate surface of the optical disc inclines to the normal substrate surface, the coma increases and consequently the reproduced signals are degraded. Inclination of the optical disc, which makes it not possible for the laser beam to perpendicularly fall upon the substrate surface, is sometimes caused by warping of the optical disc or the like. On the other hand, as the coma is also proportional to the thickness of the optical disc substrate, it is possible to control the coma due to the inclination of the optical disc substrate by reducing the thickness of the optical disc substrate. Based on this principle, to reduce the spot diameter of the laser beam to approximately 0.9 .mu.m by increasing the numerical aperture NA of the objective lens while controlling the coma due to the inclination of the optical disc substrate, a means for reducing the thickness of the optical disc substrate has been examined.
The objective lens of the optical pickup is designed considering the thickness of the optical disc and the wavelength of the laser beam. Therefore, if the substrate thickness of the optical disc on which information is recorded or from which information is read is different from the substrate thickness supposed to be in the objective lens design process, wave aberration results. As a result, the laser beam fails to focus upon the information-bearing surface of the optical disc, and information recording or information reproducing to or from the optical disc is not possible. For example, when an optical pickup mounted with an objective lens designed for an optical disc of 0.6 mm in substrate thickness is used, it is not possible to focus the laser beam upon the information-bearing surface of an optical disc of 1.2 mm in substrate thickness, and therefore, it is not possible to record the information on or read the information recorded on the optical disc of 1.2 mm thickness. This means, for two different types of optical discs with different substrate thicknesses, an optical pickup mounted with two objective lens suitable to the respective different types of optical discs must be prepared.